The research in this proposal deals with the characterization of genetic and biochemical mechanisms responsible for the regulation of replication and for stable maintenance of the broad-host-range plasmid RK2 in bacteria. This plasmid specifies resistance to the antibiotics tetracycline, ampicillin and kanamycin and is stably maintained in the extrachromosomal state in a wide range of Gram-negative bacteria. Two components of this plasmid that are absolutely required for its replication are the trfA gene that encodes two replication initiation proteins (44 kDa and 33 kDa), the smaller of which is the result of an internal translational start in the same open reading frame, and a replication origin sequence that contains as its main feature eight 17 bp repeats (iterons) in groups of five and three. The 44 kDa and 33 kDa proteins are essentially equivalent in activity in the bacterium Escherichia coli and both proteins specifically bind to the iterons at the RK2 replication origin. The major thrust of this proposal is to examine the nature of the interactions between the TrfA replication protein(s) and the sequence at the replication origin that are responsible for the regulation of initiation of replication and the ability of this antibiotic resistance plasmid to be maintained in a wide range of bacteria. These studies also will be concerned with analyses of the interactions between the TrfA protein/origin sequence complex and specific host replication proteins isolated from several Gram-negative bacteria. The analyses will be carried out in vivo and in vitro and will involve both wild-type and mutant TrfA proteins, including defective and copy-up mutants. An attempt will be made to identify regions of the TrfA protein responsible for its various interactions with the replication origin and the host replication proteins. Finally, a region of the RK2 plasmid that appears to encode a broad-host-range plasmid partitioning function will be examined in order to understand the mechanism(s) responsible for the partitioning of a plasmid to daughter cells upon cell division. In addition to providing information on the regulation of initiation of replication and the stable maintenance of an extrachromosomal element in bacteria, a practical fallout of this basic study will be the construction of high-copy number and stably maintained plasmid vectors for gene cloning in a wide range of Gram-negative bacteria of medical and agricultural importance.